1. Field of the Invention
The present invention relates to power tools such as fastener driving devices.
2. Description of Related Art
Fastening tools are designed to deliver energy stored in an energy source to drive fasteners very quickly. Typically fastener driving devices use energy sources such as compressed air, flywheels, and chemicals (fuel combustion & gun powder detonation). For some low energy tools, steel springs are used. For example, U.S. Pat. No. 6,899,260 discloses a small cordless brad tool. U.S. Pat. No. 6,997,367 discloses a hand held nailing tool for firing small nails.
It is desirable for the tool to be of low weight so that it may be used with one hand, and not cause excessive fatigue. It is also desirable for fastener driving devices to provide sufficient energy to effectively drive the fastener, but with minimum recoil. Recoil negatively impacts a tool's ability to drive a fastener, and, it may also increase user fatigue.
Recoil is a function of, among other things, the tool weight/driver weight ratio, and driver velocity or drive time. As a fastener is being driven, a reaction force is pushing the tool off of the work surface. The distance the tool moves off of the workpiece is proportional to the drive time and other parameters as noted herein below. A typical pneumatic tool has a tool/driver ratio of greater than 30. Drive time is typically less than 10 milliseconds (msec.) and should not be greater than 20 msec., and preferably, not be greater than 15 msec. Maximum pneumatic tool weight is found with the bigger tools—e.g., framing nailers. An estimated maximum limit to an acceptable tool weight is 10 lbs. Framing nailers in the 8 to 9.5 lb. range are typically used without excessive fatigue. Combining the limits on the tool/driver weight ratio of 30 and a 10 lb. maximum tool weight, the limit on the driver weight becomes about 0.33 lb. That is, the driver weight should preferably be less than 0.33 lb. if the tool weighs 10 lbs. In other words, if the driver (mechanism in the tool that drives the fastener) weighs more than 0.33 lb., the tool weight would have to be greater than 10 lb. to counteract the recoil sufficiently for comfortable operation and adequately drive the fastener into the workpiece in a single blow.
Another reason for the quick drive time requirement is the dual requirement of energy and force. The energy is stored in a moving mass and can be found from Energy=½ mass×velocity squared, i.e. E=½mv2. An impulse force is developed from the change in momentum when the driver pushes the fastener into the work piece. Assuming an average force during the drive and the final velocity of the moving driver mass is zero, a simple equation may be set up where force×time=mass×velocity, or time=mass×velocity/force.
In general, the event of driving most fasteners in a single drive stroke occurs in fewer than 10 msec., which would allow for a rate of 100 cycles per second. Of course, this time does not take into consideration the reset time. Pneumatic tool cycle rates typically range from approximately 30 cycles per second for very small energy tools such as upholstery staplers, to approximately 10 cycles per second for larger energy tools, for example, tools that are used in framing. In most applications, the desired rate is no more than 10 cycles per second, which allows for 100 msec. per actuation.
The constraint of the drive time being less than 10 msec. is still desirable to minimize the recoil of the tool and to adequately drive the fastener, as previously described. Of course, these factors are inter-related in that if the tool does not adequately drive the fastener, recoil will typically be more severe. As stated above, recoil is a function of many things, but a primary physical consideration is the ratio between the tool weight and the weight of the driver. This is due to the energy requirement of driving a fastener being constant. Also, the law of conservation of momentum requires that the final velocity of the tool (assuming the tool velocity is zero at the start) will be equal to the ratio between the mass of the tool and the mass of the driver times the final velocity of the driver. The output energy of the tool (when no fastener is driven) is equal to ½ the mass of the driver times the square of the final velocity of the driver (½×m×v2). Combining these two principles and simplifying, the final velocity of the tool may be found from Equation 1:
                              V          tool                =                                            2              ⁢                              m                striker                            ⁢                                                          ⁢              Energy                                      m              tool              2                                                          (        1        )            
Holding the mass of the tool and energy constant, the only practical way to decrease the tool velocity from Equation 1 is to decrease the mass of the driver. As the driver gets lighter, its final velocity has to increase to maintain the required energy. Given that time is equal to distance divided by velocity, and assuming that average velocity is about half peak velocity for most single stroke fastener drive events, the optimal and practical time to drive a fastener in a single drive stroke is between 3 and 10 msec.
One problem with a short drive time is the high power requirement it creates. Given that power is output energy divided by time, as the time decreases for a given energy, the power increases. Although most applications allow 100 msec. per actuation, an improved drive allows 10 msec. or less, and realizes at least a 10 fold increase in power. This creates the need for some sort of energy storage device that can release or transfer it's stored energy in 10 msec., or less.
Direct chemical energy can be released in less than 10 msec., but direct chemical energy in discrete actuations has other costs and complexities that make it limited at the present time (e.g. fuel cost, exhaust gases). However, chemical energy based tools typically cannot practically provide “bump fire” capability where the trigger is depressed, and the contact trip is depressed to start a drive sequence. Another form of energy storage that allows for the storage and rapid release of energy is the flywheel. Mechanical flywheel type cordless fastening tool proposed in U.S. patent application US20050218184(A1) maintains a constant flywheel speed, while the tool proposed in U.S. Pat. No. 5,511,715 does not maintain a constant flywheel speed. However, one recognized problem with a flywheel is long term energy storage, which creates a need to get the total required energy for a first actuation into the flywheel before the perceived actuation delay time which is approximately 70 msec. In particular, from a user's perspective, the maximum delay from when the contact trip is depressed, to when the nail is driven, is approximately 70 msec. Tools having larger actuation delay time will typically be deemed unacceptable for use in bump fire mode. In addition, when a tool is bumped against the work surface to drive a fastener, the tool naturally begins to bounce off the surface, and after approximately 70 msec. has lapsed, the tool may have moved far enough away from the workpiece to prevent complete driving of the fastener into the workpiece. Thus, flywheel based tools must maintain constant rotation of the flywheel while the trigger is depressed to have such bump fire capability, thus wasting energy to maintain the flywheel speed. Another problem with a flywheel is the energy transfer mechanism is complicated and inefficient.
Other devices peripherally related to the fastener driving devices are disclosed in U.S. Pat. No. 5,720,423 that provides a discussion as to why a traditional steel spring cannot be effectively used to drive a nail, U.S. Pat. No. 7,137,541 that discloses a cordless fastener driving device with a mode selector switch, and U.S. Pat. No. 3,243,023 that discloses a clutch mechanism. Moreover, various references related to coil springs in general, are known.
However, there still exists an unfulfilled need for a lightweight and efficient fastener driving device that provides sufficient energy to drive a fastener. There also exists an unfulfilled need for such a fastener driving device that allows bump fire actuation.